1. Field of the Invention
This invention relates to a viewfinder of the real image type.
2. Description of the Prior Art
A camera of the lens shutter type which is compact in size and low in cost conventionally employs an inverted Galilean viewfinder of the virtual image type. A conventional viewfinder of the Albada type (inverted Galilean type) which is used frequently for cameras of the lens shutter type is schematically shown in FIG. 20. Referring to FIG. 20, the viewfinder shown includes an objective 101 and an eyepiece 102 which are disposed such that a virtual image of an object for photographing may be made at a position spaced by about one meter or so from a photographer so as to enable the object to be observed by the photographer. In order to enable a photographing range or a distance measuring range to be observed together with such object by the photographer, a virtual image of a pattern of a photographing range or a distance measuring range which is drawn on a transparent member (field frame) 103 is made, using a reflecting mirror 104 and the eyepiece 102, at a position substantially the same as the position at which the virtual image of the object is made. Such a viewfinder optical system as described just above, however, is disadvantageous in that when an object is very bright or very dark, the photographing range or distance measuring range cannot be observed readily and additionally the photographer cannot observe the photographing range and the distance measuring range clearly because the diopter of the eyepiece 102 does not coincide with the field frame 103.
For indication of exposure information or some other information in such an inverted Galilean viewfinder as described above, it has been proposed to cause a light source to emit light in a light path in the viewfinder. FIG. 21 schematically shows such a proposed viewfinder wherein a light source 106 such as a light emitting diode is disposed between a reflecting mirror 104 and an eyepiece 102. With the viewfinder, however, since the light source 106 is not positioned at a focus of an eye of a photographer, information cannot be discriminated depending upon a configuration but must be discriminated only depending upon a color of the light source 106. Accordingly, the viewfinder is disadvantageous in that indication only of a limited amount of information can be attained and contents of such information cannot be readily recognized visually.
Another proposal has been made to improve such disadvantages of the conventional viewfinder as described just above. Such proposed viewfinder is schematically shown in FIG. 22 and includes a mask member 107 which is positioned at a diopter conforming to a field frame 103. Light is projected from a light source 106 located at the back of the mask member 107 so as to indicate exposure information or some other information as an image to a photographer. The improved viewfinder, however, is still disadvantageous in that, where such image is indicated within a field limiting frame 105, if the ambient light is very bright, the image cannot be observed clearly. Further, if it is attempted to indicate such information image outside the field limiting frame 105, then the size of the viewfinder must be increased as much. Besides, since the diopter does not coincide with the field limiting frame 105, such information image cannot visually look very fine.
Such disadvantages of the conventional viewfinders as described above can be eliminated where an optical system of the real image type is employed for a viewfinder. An exemplary one of such viewfinders is such a Keplerian viewfinder which employs a Porro prism as shown in FIG. 23. Referring to FIG. 23, the viewfinder shown includes an objective 101 in the form of a convex lens, a field frame 103, a Porro prism composed of two parts 108 and 109, and a pair of eyepieces 102a and 102b cooperating with each other to form a convex lens. In principle, an image made by the objective 101 is enlarged by the eyepieces 102a and 102b to facilitate observation thereof by a photographer. If the field frame 103 is placed at a position at which an image is to be formed by the objective 101, then a photographer can clearly observe both of the object image and the field frame.
With the Keplerian viewfinder, however, where the Porro prism is made of glass, they require a comparatively high production cost and have a comparatively great weight. To the contrary, where the Porro prism is made of a plastic material, they can be produced at a reduced cost, but they often have residual internal strains and are thus inferior in image forming performance.
In order to resolve the problem, it may seem recommendable to provide mirrors corresponding to reflecting faces of such Porro prism. The arrangement thus requires up to four mirrors and may require an independent mirror holder for each of such mirrors, resulting in an increase in number of parts. Also, a mechanism for adjustment of each of the mirrors in position is required, which will cause a further rise in production cost. Meanwhile, if dust should enter to a position at which an image is to be formed by the objective, it will be enlarged by the eyepiece and distrub a field of view of the photographer. Therefore, a dust-proof countermeasure is required. Where the four mirrors are held independently of each other, however, it is difficult to maintain an enclosed condition of the viewfinder.